Organohalosilanes, halosilanes, and in particular methylchlorosilanes, are the building blocks from which silicone polymers are produced. Organohalosilanes and halosilanes are commercially produced by what is commonly called “the direct process”, in which silicon metal is reacted with an organic halide or hydrogen chloride, optionally in the presence of a catalyst. The direct process is well known in the art, and is well described in patent literature.
In, for example, the commercial production of methylchlorosilanes by the direct process, finely ground silicon metal powder is reacted with methyl chloride in the presence of a catalyst by fluidising the silicon powder in a fluid bed by passing methyl chloride gas there through at a temperature of between 200° C. and 500° C. At the end of the reaction, there remains in the fluid bed spent bed material. Similar steps are taken and similar spent bed material is produced when manufacturing halosilanes.
In the commercial production of methylchlorosilanes, after the removal of the methylchlorosilanes from the fluid bed reactor, the spent bed material remaining in the fluid bed reactor comprises mainly silicon metal and silicon oxides, with minor amounts of other materials such as catalyst (if used), unreacted organic halide, carbon residues, impurities and reaction product.
It is well known that spent bed material can be hazardous if left in an unpacified state. When in contact with moisture, such as atmospheric moisture, it evolves hydrogen gas, and it is highly susceptible to atmospheric oxidation, i.e. it is self-heating, and represents a potential fire hazard. Consequently, spent bed material is commonly pacified through quenching by reaction with an alkaline substance, for example an aqueous lime solution, with heating.
A typical quenching method is described in EP 428,337 (Dow Corning) in which spent bed material is heated in an aqueous basic solution with stirring, and then separated from the heated aqueous basic solution by filtration, pressed into cakes, and aged to reduce any residual chemical activity. Such quenching processes are however time consuming, the basic solution typically contacting the spent bed material for 6 to 12 hours whilst it neutralises and converts metallic silicon to silica. After neutralisation, the solid bed material has to separated by filtration and the aqueous filtrate invariably contains some metals, which have come from the spent bed. These have to be removed by water treatment steps before the filtrate can be discharged into the environment. After filtration, a substantial amount of water remains with the solid spent bed material, which adds considerably to the weight of material which has to be transported. Thus, quenching methods tend to be economically expensive.
Russian Patent No. 2118561 (State Scientific Centre of the Russian Federation) describes a method for granulating hard waste resulting from the synthesis of organochlorosilanes. Hard waste is granulated into hard granules (e.g. about 5-40 mm in size) by mixing with an aqueous agent comprising from 5 to 40 weight % of alkaline agent selected from sodium hydroxide, sodium silicate, calcium hydroxide and mixtures thereof. Granulation is carried out in a suitable granulator such as a rotary drum granulator and the resulting granules are calcined. The passivated granulated material is described as being useful for repeated use of copper and silicon in copper metallurgy, for restorative smelting of silicon and for metallurgic processes as the source of ferrosilicon, copper etc.
In order to aid granulation, connective substances such as bentonite, lime, opoka, dolomite fluor, clay or mixtures thereof can be added to the mixture being granulated. In example 5, bentonite, and in example 7, clay, are added to the granulator in maximum amount of 8.8 and 8.1 weight %, respectively, of the total material added to the granulator. Even if all the water is removed during drying and calcination, the maximum bentonite and clay contents in the product of examples 5 and 7 is 9.7 and 8.9 weight %, respectively.
The process described in RU 2118561 is still time consuming and expensive. It still involves the use of an aqueous basic solution and results in much of the silicon metal being converted into silica. Furthermore, it necessitates that the granules be calcined.
The present inventors have found an economically advantageous way to pacify spent bed material, which method also provides an industrially useful end product. They have found that mixing spent bed material with clay pacifies the spent bed material and that the resulting clay composition is non-self heating and evolves substantially reduced amounts of hydrogen gas compared to spent bed material alone. The method is relatively quick with the processing time from fresh feeds to a pacified product typically being less than 10 minutes. Furthermore, the method does not require the use of an aqueous basic solution and the water content of the pacified material is low which reduces transport costs and processing costs for end uses. Unlike prior art processes, in the process of the present invention, all or most of the silicon metal content of the spent bed is preserved, the properties of which can be utilised. In addition, it is not necessary to calcinate the pacified material of the present invention.